Self-aligned source and drain regions for semiconductor devices

ABSTRACT

A method for forming a semiconductor device includes patterning a gate conductor, formed on a substrate, and a two-dimensional material formed on the gate conductor. Recesses are formed adjacent to the gate conductor in the substrate, and a doped layer is deposited in the recesses and over a top of the two-dimensional material. Tape is adhered to the doped layer on top of the two-dimensional material. The tape is removed to exfoliate the doped layer from the top of the two-dimensional material to form source and drain regions in the recesses.

BACKGROUND

Technical Field

The present invention relates to semiconductor devices and processes,and more particularly to semiconductor devices that employ depositionand exfoliation of layers to form source and drain regions fortransistor devices.

Description of the Related Art

Field effect transistors (FETs) often include doped source and drainregions made of a similar material. In one common structure, III-V FETsinclude source/drain (S/D) regions formed from doped InGaAs (e.g., n+InGaAs). The formation process for forming S/D regions usually requirespatterned implantation of n+ dopants, which adds time and expense to theprocess, and may result in junction damage. To avoid ion-implantation,selective epitaxial growth processes may be employed.

The selective epitaxial growth processes for FET structures often form agate structure (with dielectric sidewall spacers on sides of a gateconductor material) over a substrate, recess regions adjacent to thegate structure and perform a selective epitaxy process to form sourceand drain (S/D) regions within the recess regions. The S/D regions aredoped using a doped layer that is deposited over the gate structure.Dopants from the doped layer are diffused into the S/D regions. Then, achemical-mechanical polish (CMP) is performed to remove the doped layerfrom a top of the gate structure. This procedure avoids ion-implantationfor doping but requires many additional steps. Selective epitaxialgrowth limits the process flow and may be expensive and time consuming.Extra CMP steps also add additional time delays and run the risk ofdamage or other issues to the already formed components on the device.

SUMMARY

A method for forming a semiconductor device includes patterning a gateconductor, formed on a substrate, and a two-dimensional material formedon the gate conductor. Recesses are formed adjacent to the gateconductor in the substrate, and a doped layer is deposited in therecesses and over a top of the two-dimensional material. Tape is adheredto the doped layer on top of the two-dimensional material. The tape isremoved to exfoliate the doped layer from the top of the two-dimensionalmaterial to form source and drain regions in the recesses.

Another method for forming a semiconductor device includes depositing agate conductor on a substrate; forming a two-dimensional material on thegate conductor; patterning the gate conductor and the two-dimensionalmaterial; etching self-aligned recesses adjacent to the gate conductorin the substrate; depositing a doped layer in the recesses, alongsidewalls of the gate conductor and over a top of the two-dimensionalmaterial; adhering tape to the doped layer on top of the two-dimensionalmaterial; and removing the tape to exfoliate the doped layer from thetop of the two-dimensional material to form source and drain regions inthe recesses.

A semiconductor device includes a gate conductor formed on a substrate.A deposited doped layer is formed in recesses adjacent to the gateconductor and extends along sidewalls of the gate conductor to formvertical portions. The doped layer forms source and drain regions forthe semiconductor device. The gate conductor is shaped between thevertical portions to permit formation of a dielectric material betweenthe gate conductor and the vertical portions.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a partially fabricated field effecttransistor with a gate conductor formed on a substrate and atwo-dimensional material formed on the gate materials in accordance withthe present principles;

FIG. 2 is a cross-sectional view of the partially fabricated fieldeffect transistor of FIG. 1 with the gate conductor and thetwo-dimensional material patterned to form a gate structure inaccordance with the present principles;

FIG. 3 is a cross-sectional view of the partially fabricated fieldeffect transistor of FIG. 2 with the gate structure self-aligning anetching process to form recesses adjacent to the gate structure inaccordance with the present principles;

FIG. 4 is a cross-sectional view of the partially fabricated fieldeffect transistor of FIG. 3 with a doped layer deposited over the gatestructure and in the recesses to form self-aligned source and drainregions adjacent to the gate structure in accordance with the presentprinciples;

FIG. 5 is a cross-sectional view of the partially fabricated fieldeffect transistor of FIG. 4 with a tape applied to a portion of thedoped layer deposited over the gate structure and employed to exfoliatethe doped layer using the two-dimensional layer as a splitting point inaccordance with the present principles;

FIG. 6 is a cross-sectional view of the partially fabricated fieldeffect transistor of FIG. 5 with the gate conductor etched to form a gapfor a dielectric spacer between the vertical portions of the doped layerand the gate conductor in accordance with the present principles; and

FIG. 7 is a block/flow diagram showing methods for forming asemiconductor device in accordance with illustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, structures and methods forformation are described for field effect transistors. In accordance withparticularly useful embodiments, two-dimensional (2D) materials areemployed in forming a gate structure. The 2D material is later employedto exfoliate a dopant rich layer used to form source and drain (S/D)regions. The dopant rich layer is formed by deposition and is employedas the S/D regions. By providing the dopant rich layer for as the S/Dregions, selective epitaxial growth is eliminated from the process flow.Selective epitaxial growth provides many limitations to the processflow, and in accordance with the present principles, these limitationsare avoided. With the use of a 2D exfoliation layer, the need to performadditional chemical mechanical polishing (CMP) to remove the dopant richlayer and other steps are eliminated as well.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, materials and process features and steps maybe varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip in accordance with the presentprinciples may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., InGaAs. These compounds may includedifferent proportions of the elements within the compound, e.g., InGaAsincludes In_(x),Ga_(1−x)As, where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound, such as, e.g.,AlInGaAs, and still function in accordance with the present principles.The compounds with additional elements will be referred to herein asalloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a cross-section of apartially fabricated field effect transistor (FET) 10 is shown inaccordance with one illustrative example. The FET 10 includes asubstrate 12. In one embodiment, the substrate 12 may include a III-Vmaterial, silicon material, germanium or other suitable substrate orwafer material.

The FET 10 is processed by forming a gate dielectric 14, e.g., SiO₂, orother dielectric material. A gate electrode layer 16 is then formed. Thegate electrode 24 may include any suitable highly conductive material,e.g., Cu, Cr, Au, Ag, W, doped polysilicon, etc.

A two-dimensional material 18 is formed on the gate conductor 16. Thetwo-dimensional (2D) material 18 includes strong bonds in two dimensionsand weak bonds in a third dimension. A 2D material may include weak Vander Waals forces perpendicular to a layer (weak vertical bonding) suchthat the material separates easily along atomic layers or strata (e.g.,strength in 2D directions). Such 2D materials can be employed asinterlayers to facilitate layer transfer of subsequently grownsemiconductor films.

The gate conductor 16 should be capable of offering locations for singlecrystalline deposition or formation of a 2D material 18 (e.g., singlecrystalline or single sheet (e.g., graphene) deposition). The 2Dmaterial of layer 18 may be deposited (e.g., epitaxially grown) on thegate conductor 16 or may be transferred to the gate conductor 16 by alayer transfer process. The layer 18 may include graphene or other 2Dmaterials, such as, e.g., pantacene, MoS₂, WS₂, boron nitride, mica,dichalcogenides and complex oxides.

Referring to FIG. 2, a gate stack 24 is patterned by etching the layers18, 16 and 14 in accordance with a mask (not shown). The mask mayinclude a photolithographic mask or other etch mask formed and developedover the 2D layer 18. Depending on material selection, a same mask maybe employed for etching all layers 18, 16, 14; however, etch chemistriesmay be altered. The substrate 12 is cleaned in exposed areas to providelocations for source and drain (S/D) region formation adjacent to thegate stack 24.

Referring to FIG. 3, the same mask employing in FIG. 2 may be utilizedto further etch recesses 20 into the substrate 12 adjacent to the gatestack 24. Alternately, a new mask may be employed or the 2D material 18may be employed as an etch mask to form recesses 20. Recesses 20 areformed to provide locations for forming source and drain (S/D) regions26 (FIG. 4). The recesses 20 are self-aligned to the gate stack 24.

Referring to FIG. 4, a dopant rich layer or doped layer 22 is depositedover the gate structure 24 on the 2D material 18 and formed in therecesses 20 to form S/D regions 26. The doped layer 22 may include asemiconductor material, such as a III-V semiconductor material (e.g.,InGaAs, InP, GaAs, etc.) or a IV semiconductor material (e.g., Ge,etc.). The doped semiconductor material may be single-crystalline,poly/multi-crystalline, micro/nano-crystalline, amorphous orcombinations thereof.

In one embodiment, doped layer 22 includes a doped Si based material, adoped Ge based material, a doped III-V material or an alloy of thesematerials. The doped layer 22 may be formed using a chemical vapordeposition (CVD) process, epitaxy, sputtering, atomic layer deposition(ALD), metal organic chemical vapor deposition (MOCVD) or other process.In one particularly useful embodiment, the deposition process forms thedoped layer 22 conformably so that the doped layer 22 forms on sidewallsof the gate conductor 16 and adjacent to the 2D material 18 formed onthe gate conductor 16.

Doped layer 22 may include n-dopants or p-dopants as needed. The dopantsmay be included during the formation of layer 22 to avoidion-implantation processes for doping S/D regions 26. In someembodiments, the dopant concentrations may be adjusted using additionalprocessing, e.g., ion implantation, applying additional doped layers anddiffusing dopants therefore into the S/D regions 26, etc.

In accordance with the present principles, the S/D/ regions 26 areself-aligned and may be formed by selecting any number of depositionprocesses. The present principles offer a degree of freedom for the typeof deposition process selected for the formation of the S/D regions 26.By depositing the material for S/D regions 26 in the self-alignedrecesses 20, selective epitaxial deposition may be avoided as well asthe CMP process steps associated with post processing of the selectiveepitaxial growth deposition process.

With the freedom of selection of the deposition process also comes theselection of the type or phase of the material forming the S/D regions26. For example, material for S/D regions 26 may be deposited or grownby one or more of the following processes, epitaxy, sputtering, atomiclayer deposition (ALD) and metal organic chemical vapor deposition(MOCVD). ALD may form the S/D regions 26 in crystalline form. The S/Dregions 26 may include a monocrystalline structure or may include amulti-crystal structure or other crystalline structure (micro, nano,etc.). However, the material of layer 22 may also include amorphousphases.

Referring to FIG. 5, a handle substrate or tape 30 is adhered to layer22 at a top portion of the gate structures 24. The handle substrate ortape 30 adheres to a horizontal portion 23 or doped layer 22. The handlesubstrate or tape 30 may include any suitable material to provideleverage for exfoliating the layer 18. The handle or flexible substrate30 may include a polymeric material(s), such as, e.g., thermoplastics,e.g., polyethylene terephthalate (PET), polyimide, etc., reinforcedepoxy resins, such as e.g., prepreg boards, etc. The flexible substrate30 may be glued or otherwise adhered by an adhesive or adhering layer tothe horizontal portion 23 of layer 22 over layer 18.

Next, a lift off or removal process is performed to remove thehorizontal portion 23 of layer 22 by exfoliating 2D layer 18. The handlesubstrate or tape 30 is stripped off of the gate structure 24. Corners25 (FIG. 4) of the layer 22 provide locations of high mechanical stress.The corners 25 may be designed to impart a high stress to the layer 22,e.g., sharp corners, abrupt changes, etc.). When the tape 30 is removed,the adhesive pulls the horizontal portion 23 of the layer 22, theportion 23 yields at the corners 25 to permit removal of the portion 23.The bonding force between the 2D layer 18 and its adjacent layers 16 or22 (portion 23), is weak. Therefore, the tape 30 will remove portion 23and the entire layer 18 or a portion of layer 18. Layer 18 may becompletely removed from the gate conductor 16 or may by split to dividemonolayers of layer 18. Depending on the energies and the bonding, thelayer 18 may be split to provide zero or more monolayers of the layer 18on the layer 16 and zero or more monolayers of the layer 18 on theportion 23. Any material from layer 18 may be cleaned from layer 16after the exfoliation (e.g., by etching).

In one embodiment, graphene is employed for layer 18, and the thicknessof the graphene layer 18 is preferably one or more monolayers formed asa single crystal or single sheet. In useful embodiments, the number ofmonolayers of graphene may be determined depending on what is needed tocleanly separate the graphene to produce a split.

Once the portion 23 is removed, the remaining portions of layer 22 maybe processed such that the layer 22 forms S/D regions 26. Verticalportions 28 of layer 22 may be employed as contacts for connecting theS/D regions 26 to higher level metallizations.

Referring to FIG. 6, once the portion 23 is removed from the gateconductor 16, the gate conductor 16 is etched to reduce its size betweenvertical portions 28. Etching the gate conductor 16 forms a trapezoidalshape to isolate the gate conductor 16 from the vertical portions 28.The etching process may include a selective etch that removes remnantsof layer 18 and etches the gate conductor 16 selectively with respect tothe layer 22 (S/D regions 26 and vertical portions 28). In this way, adielectric material 32 may be deposited over the gate conductor 16 (andpossible other regions) to isolate the gate conductor 16 from thevertical portions 28. Dielectric material 32 may include an oxide,nitride or any other suitable dielectric material. Processing cancontinue with the formation of metallizations and other structures as isknown in the art.

It should be understood that the present principles have beenillustratively demonstrated using a transistor structure; however, thestructure and method steps may be employed with any electronic device,including diodes, lasers, passive elements (e.g., inductor, capacitors,resistors), junctions, solar cells, etc. The present principles may beemployed on any devices or components that may have a depositedstructure with a portion removable with a 2D material exfoliation.

Referring to FIG. 7, a method for forming a semiconductor device isshown in accordance with illustrative embodiments. In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In block 102, a gate conductor is deposited on a substrate. The gateconductor may include a gate metal, such as e.g., Cu, Ag, Au, Pd, Ti, W,etc. The gate conductor may include multiple layers. The substrate mayinclude, e.g., a III-V substrate, a Si substrate, a Ge substrate, asilicon on insulator substrate, etc. The III-V materials may include,e.g., InP, InAs, AlAs, AlN, GaN, InN, AlP, GaP, InP, AlAs, GaAs, InAs,etc. or tertiary compounds, e.g., InGaAs, AlGaAs, etc.

In block 104, a two-dimensional material is formed on the gateconductor. The two-dimensional material may be formed by deposition orlayer transfer. The two-dimensional material may include one or more of:e.g., graphene, pantacene, MoS₂, WS₂, boron nitride, mica,dichalcogenides, a complex oxide, etc.

In block 106, the gate conductor and the two-dimensional material aredefined by patterning. The gate conductor and the two-dimensionalmaterial may be etched in accordance with a same mask, although multiplemasks may be employed. Different etch chemistries may be employed toetch down to the substrate.

In block 108, self-aligned recesses are etched adjacent to the gateconductor into the substrate. In block 110, a doped layer is depositedin the recesses and over a top of the two-dimensional material. Thedoped layer may also be deposited along sidewalls of the gate conductor.The doped layer includes a material compatible with the substrate. Thedoped layer may include III-V materials, e.g., InP, InAs, AlAs, AlN,GaN, InN, AlP, GaP, InP, AlAs, GaAs, InAs, etc. or tertiary compounds,e.g., InGaAs, AlGaAs, etc., IV materials, e.g., Si, Ge, SiGe, etc.

The deposition process for forming the doped layer may be selected fromany of a plurality of deposition processes. In particular, depositingthe doped layer may include forming the source and drain regions by aprocess other than selective epitaxial growth, e.g., employing an ALDprocess, MOCVD, sputtering, CVD, etc.

In block 112, tape (or other substrate) is adhered to the doped layer ontop of the two-dimensional material. In block 114, the tape is removedto exfoliate the doped layer from the top of the two-dimensionalmaterial to form source and drain regions in the recesses. In oneembodiment, the doped layer forms corners over the sidewalls of the gateconductor adjacent to the two-dimensional material. The corners provideyield points where the doped layer breaks to exfoliate the portion ofthe doped layer on a top of the gate structure. In one embodiment, thedoped layer may remain on the sidewalls of the gate conductor afterremoving the tape from the top of the two-dimensional material.

In block 116, the gate conductor is etched to form a gap between thesidewalls of the gate conductor and vertical portions of the doped layeralong the sidewalls of the gate conductor. The etch may be employed toclean off any remaining two-dimensional material. In block 118, the gap(between the sidewalls of the gate conductor and vertical portions ofthe doped layer) is filled with a dielectric material between thesidewalls of the gate conductor and the vertical portions of the dopedlayer. In block 120, processing continues to complete the device. Thedevice may include a transistor, diode, inductor, resistor, capacitor,solar cell, etc. Further processing may include forming metallizations,etc.

Having described preferred embodiments for self-aligned source and drainregions for semiconductor devices (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method for forming a semiconductor device,comprising: depositing a doped layer over a top of a two-dimensionalmaterial formed on a gate conductor, wherein the gate conductor isformed on a substrate; adhering tape to the doped layer on top of thetwo-dimensional material; and removing the tape to exfoliate the dopedlayer from the top of the two-dimensional material to form source anddrain regions.
 2. The method as recited in claim 1, further comprisingforming the two-dimensional material on the gate conductor by one of adeposition process or a transfer process.
 3. The method as recited inclaim 1, wherein depositing the doped layer includes depositing thedoped layer on sidewalls of the gate conductor.
 4. The method as recitedin claim 3, wherein the doped layer remains on the sidewalls of the gateconductor after removing the tape to exfoliate the doped layer from thetop of the two-dimensional material.
 5. The method as recited in claim1, further comprising etching the gate conductor to form a gap betweenthe sidewalk of the gate conductor and the vertical portions of thedoped layer.
 6. The method as recited in claim 5, further comprisingfilling the gap with a dielectric material between the sidewalk of thegate conductor and the vertical portions of the doped layer.
 7. Themethod as recited in claim 1, wherein depositing the doped layerincludes depositing the doped layer over the top of the two-dimensionalmaterial and along sidewalk of the gate conductor to form cornersadjacent to the two-dimensional material, the corners providing yieldpoints to exfoliate the doped layer.
 8. The method as recited in claim1, wherein the two-dimensional material includes one of: graphene,pantacene, MoS₂, WS₂, boron nitride, mica, dichaicogenides or a complexoxide.
 9. The method as recited in claim 1, wherein depositing dopedlayer includes forming the source and drain regions by a process otherthan selective epitaxial growth.
 10. A method for forming asemiconductor device, comprising: depositing a doped layer alongsidewalls of the gate conductor and over a top of a two-dimensionalmaterial formed on the gate conductor; adhering tape to the doped layeron op of the two-dimensional material; and removing the tape toexfoliate the doped layer from the top of the two-dimensional materialto form source and drain regions in recesses on opposite sides of thegate conductor.
 11. The method as recited in claim 10, furthercomprising forming the two-dimensional material by one of a depositionprocess or a transfer process.
 12. The method as recited in claim 10,wherein the doped layer remains on the sidewalls of the gate conductorafter removing the tape to exfoliate the doped layer from the top of thetwo-dimensional material.
 13. The method as recited in claim 10, furthercomprising etching the gate conductor to form a gap between thesidewalls of the gate conductor and vertical portions of the doped layeralong the sidewalls of the gate conductor.
 14. The method as recited inclaim 13, further comprising filling the gap with a dielectric materialbetween the sidewalls of the gate conductor and the vertical portions ofthe doped layer.
 15. The method as recited in claim 10, wherein thedoped layer forms corners over the sidewalk of the gate conductoradjacent to the two-dimensional material, the corners providing yieldpoints to exfoliate the doped layer.
 16. The method as recited in claim10, wherein the two-dimensional material includes one of: graphene,pantacene, MoS₂, WS₂, boron nitride, mica, dichalcogenides or a complexoxide.
 17. The method as recited in claim 10, wherein the recesses onopposite sides of the gate conductor are etched into the substratepartially below the gate conductor.
 18. The method as recited in claim17, further comprising forming a gate dielectric on the substrate,wherein the gate dielectric is between the gate conductor and thesubstrate.
 19. The method as recited in claim 18, wherein the gatedielectric is between the gate conductor and the source and drainregions in the recesses on opposite sides of the conductor.
 20. Themethod as recited in claim 18, wherein the source and drain regions aremonocrystalline structures or multi-crystal structures.